1. Field of the Invention
The present invention relates generally to metallic structures for solid oxide fuel cells, and more specifically to metallic interconnects and metallic frames and metallic frame seals for solid oxide fuel cells.
2. Technical Background
In Solid Oxide Fuel Cells (SOFCs) a solid electrolyte with permeable anode and cathode disposed on either side is exposed to oxygen (normally in air) on the cathode side and fuel such as hydrogen gas or its precursor on the anode side. Typical electrolyte materials are oxide-ion conducting. Oxygen, reduced to oxide ions at the cathode, migrates through the electrolyte to the anode to react with the fuel. The excess charge thus built up across the anode-cathode pair provides a source of electrical current. As open-circuit voltage generated across a single cathode/anode pair is in the neighborhood of 1 volt, many cathode/anode pairs are generally interconnected in series to provide industrially useful voltage and power levels. If interconnect resistivity is too high, however, a major fraction of the power generated gets converted to heat energy and is not available as electrical output.
In addition to low initial resistivity, choice of interconnect materials should be based on oxidation characteristics such that prolonged high temperature exposure of the material provides a protective oxide layer which does not grow rapidly over time and does not increase resistivity. Prolonged high temperature exposure of some metallic materials also leads to evaporation of constituent elements. This not only contributes to degradation of the interconnect materials, but also tends to contaminate the other functional components of the cell, decreasing cell performance. Hence it is also important that interconnect material is stable at operating temperatures.
SOFC interconnects thus desirably possess several properties: long-term chemical stability at high temperatures in both reducing and oxidizing environments; high electronic and low ionic conductivity in these same conditions; low contact resistance; thermal expansion closely matching that of related fuel cell components; mechanical durability through repeated temperature cycling, and of course, relatively low cost. Frame or cell wall materials for solid oxide fuel cells desirably share many of the same properties, but without a requirement of high electronic conductivity, and with a premium on thermal expansion match to the electrolyte.
Ceramic and noble-metal interconnect materials are usually prohibitively expensive for mass-produced applications. Commercially available high-chromium ferritic steels and variations thereof are the principal non-noble metallic materials that have been proposed and investigated for achieving the desired combination of properties. A chromium oxide layer forms on outer surfaces of these alloys, protecting the bulk material. The layer provides reasonably low resistivity, and some resistance to oxidizing and reducing environments. At typical fuel cell operating temperatures, however, formation of volatile chromium oxides or oxy-hydroxides or other migratory species tends to contaminate the cell, reducing the performance of the cathode and other components.
Ferritic steels with high aluminum content, that form alumina layers, while generally even more chemically stable because of the very slow growth and great durability of alumina, have typically been avoided because of the high resistivity of the alumina layer. In one exception to the general avoidance of alumina-forming alloys for fuel cell interconnect materials, ion implantation has been used as a means of increasing the conductivity of such protective alumina layers on ferritic steels that form them. Less expensive alternatives are desirable.